leetcode Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

public class addtwonum {
    public static void main(){

    }
    public ListNode addTwoNumbers (ListNode l1 , ListNode l2){
        ListNode c1 = l1 ;
        ListNode c2 = l2;
        ListNode sentinel = new ListNode(0);
        ListNode d = sentinel;
        int sum = 0;
        while (c1 != null || c2 != null){
            sum /=  10;
            if (c1 != null){
                sum += c1.val;
                c1 = c1.next;
            }
            if (c2 != null){
                sum += c2.val;
                c2 = c2.next;
            }
            d.next = new ListNode(sum % 10);
            d= d.next;
        }
        if (sum /10 ==1){
            d.next = new ListNode(1);
        }
        return sentinel.next;

    }
}
  class ListNode {
     int val;
     ListNode next;
     ListNode(int x) { val = x; }
  }